You’ve seen Hurricane Irene’s path predicted on maps: lime green states, electric blue water and a white upside-down teardrop running smack into North Carolina. But hurricanes are fickle and go where they will, so how do weather forecasters nail them down?

Actually, they don’t, which is part of the problem when you’re wrestling mathematically with a monster cyclone hundreds of miles in size. All forecasters can do is estimate with increasing uncertainty as they project forward through time where a hurricane might go. That’s what the white teardrop—sometimes called a “Cone of Uncertainty”—is all about in these National Hurricane Center maps. Don’t mistake it for something like Irene’s “area of effect,” it’s actually a zone representing the range of possible paths along which Hurricane Irene’s eye (the relatively calm, cloudless point at a hurricane’s center) could move. Think of it as a visual representation of forecasters’ margin of error.

How do forecasters determine the “Cone of Uncertainty”? According to CNN meteorologist Dave Hennen, they run simulations on “some of the fastest computers in the world,” which in turn crunch data assembled from radar, satellite and weather balloon scans, reports from ships in the vicinity of the hurricane, airplanes (hot-rod hunters that actually fly into the center of the storm) and weather stations.

“Literally billions of calculations are done with very complex equations to help model the atmosphere into the future,” Hennen says. “More than 20 different kinds of models are run – some being more reliable and complex than others – to help forecast the track and intensity of the storm.”

Forecast tracks are issued every six hours and take into account the latest data, resulting in the multicolored “spaghetti” lines you sometimes see on TV, detailing the hurricane’s possible paths, which in turn help to generate the “Cone of Uncertainty.” According to Hennen, Irene’s center location 12 hours out is averaging 36 miles in either direction, while at 48 hours out, you’re looking at a whopping 100 miles either way.

“This is why meteorologists and emergency managers will constantly preach not to look at the line on the forecast track, but to look at the ‘cone’,” Hennen says. “If you are inside that area, you could end up in the direct path of the storm.”